Create, assign and print variables in loop

[steven_tr]

I want to create and assign a value to a variables in a for loop and name the variable in the loop. This works as intended. Afterwards, I would like to print the variables something like:

a = 3
source=(0.1, 0.2, 0.3)
for n in range (a):
    exec("var"+str(n)+" = source[n]")
    print('var'+str(n))

However, I end up with:
var0
var1
var2
Whereas I want output corresponding to this:

print(var0)
print(var1)
print(var2)

How do I change the code accordingly?

[ndc85430]

You really shouldn't be trying to dynamically create variables like that. For a start, it's just over-complicated and you'll find your code becomes hard to maintain. Why exactly do you want to do it, i.e. what problem are you trying to solve?

[Knight18]

Well the reason is that "var" + str(n) creates a string, not a variable.

But yeah, you need to reconsider your approach.

[buran]

as advised - use proper data structure - list/dict, don't create names dynamically

[steven_tr]

First of all, thanks for you feedback. Since I am new to Python, there is a lot to learn:

1. Despite the fact that my proposal is not really suited, what would be the necessary adjustment such that I obtain the desired result?
   
2. I have a number of variables that may change, here "a". This number always corresponds to the number of elements in "sources". I need to create a variable that contains the value in the list "sources" and its position in the name of the variable. What would be the right way to solve this problem?
   

[buran]

as already said - use proper data structure

# sources is a tuple
sources=(0.1, 0.2, 0.3)

# access elements in tuple/list by index
print(sources[0])
print(sources[2])

# you can assign to a name
foo = sources[1]
# use that name
print(foo)


# using dict, created with dict comprehension
spam = {f'a{idx}':num for idx, num in enumerate(sources)}
print(spam)
# access elements by key
print(spam.get('a0'))
print(spam['a1'])
print(spam.get('a5', 'wrong key'))


# using dict, created another way
spam = {}
for idx, num in enumerate(sources):
    spam[f'a{idx}'] = num 
print(spam)
# access elements by key
print(spam.get('a0'))
print(spam['a1'])
print(spam.get('a5', 'wrong key'))

Output:
0.1
0.3
0.2
{'a0': 0.1, 'a1': 0.2, 'a2': 0.3}
0.1
0.2
wrong key
{'a0': 0.1, 'a1': 0.2, 'a2': 0.3}
0.1
0.2
wrong key

[steven_tr]

This may be the proper way to code the problem, but it just confuses me a lot.

Is it really not possible to use what I have suggested? It may appear wrong to you, but I would like to know it anyway.

[ndc85430]

Why does it confuse you?

It would be possible to do what you wanted, but you'd have to make more use of exec (or eval, I guess). What are the problems with that? There are at least two:

1. Those functions allow any arbitrary Python expression to be entered in a string. So, if data came in from a user, that could present a security risk (in the same way that SQL injection does).

2. You'd end up having a lot of code written in the strings you pass to those functions. That's quickly going to become unreadable as you'll lose the benefits of syntax highlighting in your editor/IDE, not to mention that you'll have lots of concatenation which you won't if you're just writing Python code in the file.

[buran]

read http://stupidpythonideas.blogspot.com/20...reate.html

still, you will need to learn how to work with container types like list/tuple/dicts, etc. it's a fundamental, you cannot progress beyond basics without it

[steven_tr]

The assignment of the values to the variables (which in fact are scalars) works just fine. I just would like to display the values of these scalars within the loop. I get it, it is wrong. Still it is what I want to do